how many people upgrade to the next model because of a couple of software updates?

Many in fact. Watch Graig's List for D7000 in the next few weeks prior to D7100 release.

I would say a very select few. Most people probably upgrade to the next model because of hardware differences not software. I really dont think this would harm their bottom line in one bit.

You don't think so but you do not know. Adding features would most likely hurt sales...by how much? Who knows. Again, it's a balancing act. I think they only did that once (with the D70 if I remember). Maybe that caused them to learn something.

Sony keeps adding features to their lower end models...and their Camera division is bleeding profit like a stuck pig.